Image processing device, display apparatus, and image processing method for converting color image signal

ABSTRACT

A decrease in a color gamut in a low gradation region is reduced to improve image quality in a liquid crystal panel. Therefore, a color image signal for a liquid crystal display panel in which a display image is generated by light passing through a rear liquid crystal cell and a front liquid crystal cell is converted into a black-and-white image signal by using a predetermined coefficient. Then, the black-and-white image signal thus obtained is subjected to gradation value conversion so as to become a signal for performing gradation expression in a gradation region set as a gradation range in which a color gamut greatly changes in a case where the front liquid crystal cell is driven by the color image signal, and thus a rear image signal serving as the black-and-white image signal for the rear liquid crystal cell is generated. Further, the color image signal is subjected to arithmetic processing by using the rear image signal, and thus a front image signal serving as the color image signal for the front liquid crystal cell is generated.

CROSS-REFERENCE TO RELATED APPLICATIONS

This present application is a continuation of U.S. Ser. No. 16/966,137,filed on Jul. 30, 2020, which is incorporated by reference in itsentirety. U.S. Ser. No. 16/966,137 is a National Stage ofPCT/JP2018/045877, filed on Dec. 13, 2018, and claims the benefit ofpriority under 35 U.S.C. § 119 of Japanese Application No. 2018-019983,filed Feb. 7, 2018.

TECHNICAL FIELD

The present technology relates to an image processing device, a displayapparatus, and an image processing method, and particularly relates toprocessing of an image signal for a display panel in which a displayimage is generated by light passing through a rear liquid crystal celland a front liquid crystal cell.

BACKGROUND ART

Liquid crystal display apparatuses having various structures are known.As one of the structures, Patent Document 1 cited below discloses adual-liquid-crystal-cell liquid crystal display apparatus.

CITATION LIST Patent Document

Patent Document 1: Japanese Patent Application Laid-Open No. 2015-191053

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

A liquid crystal cell has a small amount of light leakage due to liquidcrystal characteristics, and there is a phenomenon that a color gamut isnarrower especially in low gradations than in high gradations due tomixing of red (R), green (G) and blue (B) spectra. This reduces imagequality of an image to be displayed.

In a dual-liquid-crystal-cell display apparatus, contrast can beincreased by controlling gradations in a rear liquid crystal cell. Anobject of the present technology is to maintain a color gamut in a lowgradation region and improve image quality of an image to be displayedin a case where the dual-liquid-crystal-cell display apparatus is used.

Solutions to Problems

An image processing device according to the present technology includes:a black-and-white conversion unit that converts a color image signalinto a black-and-white image signal by using a predeterminedcoefficient, the color image signal being a signal for a display panelin which a display image is generated by light passing through a rearliquid crystal cell and a front liquid crystal cell; a gradation valueconversion unit that performs gradation value conversion so that theblack-and-white image signal obtained in the black-and-white conversionunit serves as a signal for performing gradation expression in agradation region set as a gradation range in which a color gamut greatlychanges in a case where the front liquid crystal cell is driven by thecolor image signal, and generates a rear image signal serving as theblack-and-white image signal for the rear liquid crystal cell; and afront image generation unit that performs arithmetic processing by usingthe rear image signal on the color image signal to generate a frontimage signal serving as the color image signal for the front liquidcrystal cell.

The image signals to be processed by this image processing device arefor a dual-liquid-crystal-cell liquid crystal display panel. The imageprocessing device processes an image signal for each of the front liquidcrystal cell and the rear liquid crystal cell of thedual-liquid-crystal-cell liquid crystal display panel. Meanwhile, theliquid crystal cell has a small amount of light leakage, and the colorgamut is narrower especially in a low gradation region than in a highgradation region due to mixing of R, G, and B spectra. In view of this,the image processing device generates the rear image signal thatperforms gradation expression in a gradation region (for example, thelow gradation region) in which the color gamut greatly changes in thefront liquid crystal cell that displays a color image. The front imagesignal is subjected to arithmetic processing so as to form anappropriate image when being combined with a rear image based on therear image signal.

In the image processing device according to the present technologydescribed above, the black-and-white conversion unit may convert thecolor image signal into the black-and-white image signal by performingcalculation using a coefficient calculated by using a lower limit valueof a gradation value set for each color included in the color imagesignal.

That is, first, the lower limit value of the gradation value of eachcolor (for example, R, G, or B) included in the color image signal isset. The lower limit value is a lower limit value of a gradation valueof the front image signal. By using the lower limit value, thecoefficient for converting a color image into a black-and-white image isset. The black-and-white conversion unit converts the color image signalinto the black-and-white image signal by calculation using thecoefficient.

In the image processing device according to the present technologydescribed above, the color image signal may include gradation values ofred, green, and blue, and the black-and-white conversion unit mayconvert the color image signal into the black-and-white image signal byperforming calculation using coefficients of red, green, and blue, eachof the coefficients being calculated by using a lower limit value of agradation value set for each of red, green, and blue.

That is, the lower limit values of the gradation values of R, G, and Bincluded in the color image signal are separately set. By using thelower limit values, the coefficients for the gradation values of R, G,and B for converting the color image into the black-and-white image areset. The black-and-white conversion unit converts the color image signalinto the black-and-white image signal by calculation using thecoefficients.

In the image processing device according to the present technologydescribed above, the lower limit value may be a gradation value of eachof red, green, and blue at which chromaticity changes to a predeterminedvalue in a low gradation region.

For example, as gradation chromaticity of each of red, green, and blue,the gradation value whose chromaticity value is stable in the highgradation region and becomes the predetermined value in the lowgradation region is set to the lower limit value.

In the image processing device according to the present technologydescribed above, the gradation value conversion unit may performgradation value conversion by using a lookup table in which an outputvalue is stored for an input value.

That is, a gradation value of the rear image signal is obtained byreferring to the lookup table (LUT) on the basis of a gradation value ofthe black-and-white image signal.

In the image processing device according to the present technologydescribed above, the lower limit value of the gradation value of eachcolor included in the color image signal and a target input gradation ofthe gradation value of the color image signal may be set, and the lookuptable may be set so that, when the color image signal having the targetinput gradation of a single color is input, the front image signalbecomes the lower limit value of the single color.

When the input color image signal includes the target input gradation ofeach single color R, G, or B, LUT conversion is performed so that thegradation values of the front image signal become the lower limit valuesof the respective single colors.

In the image processing device according to the present technologydescribed above, the target input gradation may be set as a maximumgradation value in a low gradation region in which human eyes hardlyperceive a change in chromaticity.

The target input gradation is a gradation in which the change inchromaticity in gradations lower than the target input gradation ishardly perceived.

In the image processing device according to the present technologydescribed above, the front image generation unit may generate the frontimage signal by dividing the color image signal by the rear imagesignal.

That is, R, G, and B gradation values of the front image signal areobtained by, for example, dividing the gradation values of R, G, and Bof the color image signal by the respective gradation values of the rearimage signal.

In the image processing device according to the above-described presenttechnology, the image processing device may further include a lightamount correction unit that multiplies the rear image signal by acorrection coefficient corresponding to a light amount componentincident on the front liquid crystal cell, and the front imagegeneration unit may generate the front image signal by dividing thecolor image signal by the rear image signal multiplied by the correctioncoefficient in the light amount correction unit.

That is, the R, G, and B gradation values of the front image signal areobtained by, for example, dividing the gradation values of R, G, and Bof the color image signal by the respective gradation values of the rearimage signal to which the correction coefficient is given in accordancewith the light amount component incident on the front liquid crystalcell.

A display apparatus according to the present technology includes: adisplay panel in which a display image is generated by light passingthrough a rear liquid crystal cell and a front liquid crystal cell; andthe above-described image processing device.

Further, in the display apparatus, the display panel may include a lightsource unit, the rear liquid crystal cell, a diffusion layer, and thefront liquid crystal cell arranged in this order.

This forms a so-called dual-liquid-crystal-cell liquid crystal displaypanel. Such a dual-liquid-crystal-cell liquid crystal display panel issubjected to image processing for reducing a change in the color gamutin the low gradation region.

In an image processing method according to the present technology, ablack-and-white conversion step, a gradation value conversion step, anda front image generation step are performed as processing performed bythe black-and-white conversion unit, the gradation value conversionunit, and the front image generation unit in the above-described imageprocessing device.

Effects of the Invention

The present technology reduces a change in color gamut caused bygradations and expands the color gamut in lower gradations, therebyimproving image quality.

Note that the effects described herein are not necessarily limited, andany of the effects described in the present disclosure may be obtained.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram of a display apparatus according to anembodiment of the present technology.

FIGS. 2A and 2B are explanatory diagrams of a front liquid crystal celland a rear liquid crystal cell according to an embodiment.

FIG. 3 is an explanatory diagram of arrangement of a liquid crystaldisplay panel according to an embodiment.

FIG. 4 is a block diagram of a dual cell image processing unit accordingto an embodiment.

FIG. 5 is a block diagram of a main part of a dual cell image processingunit according to an embodiment.

FIG. 6 is an explanatory diagram of a liquid crystal caused by lightleakage in a low gradation region.

FIG. 7 is an explanatory diagram of a relationship between gradation andchromaticity of each of R, G, and B.

FIG. 8 is an explanatory diagram of gradations used for R, G, and B inan embodiment.

FIG. 9 is an explanatory diagram of gamma of each color obtained in acase where an input gradation is set to a maximum value with gamma of2.2.

FIGS. 10A and 10B are explanatory diagrams of a lookup table settingaccording to an embodiment.

FIGS. 11A-11C are explanatory diagrams of input/output characteristicsof a rear liquid crystal cell and a front liquid crystal cell accordingto an embodiment.

FIGS. 12A and 12B are explanatory diagrams of a color gamut expansioneffect in a low gradation region according to an embodiment.

MODE FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments will be described in the following order.

<1. Configuration of display apparatus>

<2. Configuration of dual cell image processing unit>

<3. Improvement in color gamut in embodiment>

<4. Conclusion and modification example>

Note that, for the sake of description, the three primary colors of red,green, and blue are denoted by R, G, and B in alphabets, respectively.

1. Configuration of Display Apparatus

FIG. 1 illustrates a configuration of a display apparatus 90 accordingto an embodiment. The display apparatus 90 includes a liquid crystaldisplay panel 1, an image processing device 10, a front liquid crystalcell driving unit 20, and a rear liquid crystal cell driving unit 30.

The liquid crystal display panel 1 is a dual-cell liquid crystal displaypanel, and includes a front liquid crystal cell 2, a diffusion layer 4,a rear liquid crystal cell 3, and a backlight 5.

On the front side of the backlight 5, the rear liquid crystal cell 3,the diffusion layer 4, and the front liquid crystal cell 2 are arrangedin this order so as to be superimposed on each other, and a viewer seesan image displayed on the front side of the front liquid crystal cell 2.

The front liquid crystal cell 2 and the rear liquid crystal cell 3 eachform a single liquid crystal display panel. In this embodiment, however,the entire dual-liquid-crystal-cell display panel will be referred to as“liquid crystal display panel 1”.

The image processing device 10 performs signal processing of an imagesignal S1 input as a color image signal (for example, a signal in anultra high definition (UHD) format or the like) for performing displayon the liquid crystal display panel 1.

The image processing device 10 includes a display image processing unit11 and a dual cell image processing unit 12.

The display image processing unit 11 performs necessary decodingprocessing, luminance processing, color processing, resolutionconversion, and the like on the input image signal S1, and supplies aprocessed image signal Sig_in to the dual cell image processing unit 12.At least at that time, the image signal Sig_in is a color image signalindicating a gradation value of each of colors R, G, and B.

Although details will be described later, the dual cell image processingunit 12 performs processing corresponding to the dual-cell liquidcrystal display panel 1.

That is, the dual cell image processing unit 12 performs signalprocessing on the input image signal Sig_in, thereby generating andoutputting an image signal (front image signal Sig_FR) for the frontliquid crystal cell 2 and an image signal (rear image signal Sig_RE) forthe rear liquid crystal cell 3.

The front image signal Sig_FR is a color image signal includinggradation values of R, G, and B. Meanwhile, the rear image signal Sig_REis a black-and-white (grayscale) image signal including a grayscalegradation value.

The front image signal Sig_FR is supplied to the front liquid crystalcell driving unit 20. The front liquid crystal cell driving unit 20drives the front liquid crystal cell 2 on the basis of the front imagesignal Sig_FR to display a color image.

The rear image signal Sig_RE is supplied to the rear liquid crystal celldriving unit 30. The rear liquid crystal cell driving unit 30 drives therear liquid crystal cell 3 on the basis of the rear image signal Sig_REto display a black-and-white image.

FIG. 2A illustrates an example of structures of the front liquid crystalcell driving unit 20 and the front liquid crystal cell 2.

The front liquid crystal cell driving unit 20 includes a display controlunit 21, a vertical driving unit 22, and a horizontal driving unit 23,and drives the front liquid crystal cell 2 with those configurations.

The display control unit 21 supplies a control signal to the verticaldriving unit 22 and also supplies an image signal (a signalcorresponding to the gradation values of R, G, and B) and a controlsignal to the horizontal driving unit 23 on the basis of the front imagesignal Sig_FR, and performs control so that the vertical driving unit 22and the horizontal driving unit 23 operate in synchronization with eachother.

Based on the control signal supplied from the display control unit 21,the vertical driving unit 22 sequentially selects one horizontal line tobe driven for display in the front liquid crystal cell 2.

Based on the image signal and the control signal supplied from thedisplay control unit 21, the horizontal driving unit 23 generates apixel voltage for one horizontal line and supplies the pixel voltage tosubpixels 26 (26R, 26G, and 26B) for the one horizontal line selected bythe vertical driving unit 22.

In the front liquid crystal cell 2, a plurality of pixels 25 is arrangedin a matrix.

Each pixel 25 includes three subpixels 26R, 26G, and 26B.

The subpixel 26R includes a red color filter, the subpixel 26G includesa green color filter, and the subpixel 26B includes a blue color filter.

The pixel voltage is supplied from the horizontal driving unit 23 toeach of the subpixels 26R, 26G, and 26B. In addition, the subpixels 26R,26G, and 26B change light transmittance in accordance with the pixelvoltage.

FIG. 2B illustrates an example of structures of the rear liquid crystalcell driving unit 30 and the rear liquid crystal cell 3.

The rear liquid crystal cell driving unit 30 includes a display controlunit 31, a vertical driving unit 32, and a horizontal driving unit 33,and drives the rear liquid crystal cell 3 with those configurations.

The display control unit 31 supplies a control signal to the verticaldriving unit 32 and also supplies an image signal (a signalcorresponding to a grayscale gradation value) and a control signal tothe horizontal driving unit 33 on the basis of the rear image signalSig_RE, and performs control so that the vertical driving unit 32 andthe horizontal driving unit 33 operate in synchronization with eachother.

Based on the control signal supplied from the display control unit 31,the vertical driving unit 32 sequentially selects one horizontal line tobe driven for display in the rear liquid crystal cell 3.

Based on the image signal and the control signal supplied from thedisplay control unit 31, the horizontal driving unit 33 generates apixel voltage for one horizontal line and supplies the pixel voltage tosubpixels 36 for the one horizontal line selected by the verticaldriving unit 32.

In the rear liquid crystal cell 3, a plurality of pixels 35 is arrangedin a matrix.

Each pixel 35 includes three subpixels 36. Each subpixel 36 includes nocolor filter. That is, the subpixels 26R, 26G, and 26B in the frontliquid crystal cell 2 include color filters of the corresponding colors,respectively, but each subpixel 36 in the rear liquid crystal cell 3includes no color filter.

The same pixel voltage is supplied from the horizontal driving unit 33to the three subpixels 36 belonging to one pixel 35. In addition, eachsubpixel 36 changes light transmittance in accordance with the pixelvoltage.

Note that the pixel 35 of the rear liquid crystal cell 3 may include theabove three subpixels as one electrode and one pixel of a black matrix.That is, the pixel 35 may include no color filter, and, in addition, mayinclude no subpixel as to liquid crystal structural elements such as athin film transistor (TFT), a transparent electrode, wiring, and a blackmatrix. In that case, one pixel 35 corresponds to the three subpixels26R, 26G, and 26B in the front liquid crystal cell 2.

Such a rear liquid crystal cell 3 can be manufactured by omitting acolor filter forming step from a manufacturing step of a general-purposeliquid crystal display panel capable of displaying a color image. Thismakes it possible to reduce development costs and manufacturing costs ofthe display apparatus 90, as compared with a case where a dedicatedproduct is developed.

The backlight 5 of FIG. 1 emits light on the basis of a backlightcontrol signal (not illustrated). The backlight 5 is arranged on therear side of the rear liquid crystal cell 3.

The backlight 5 includes, for example, a light emitting unit using alight emitting diode (LED) to emit light.

FIG. 3 illustrates an arrangement configuration of the liquid crystaldisplay panel 1.

As illustrated in FIG. 3 , the liquid crystal display panel 1 includesthe backlight 5, the rear liquid crystal cell 3, the diffusion layer 4,and the front liquid crystal cell 2 arranged in this order, and an uppersurface of the front liquid crystal cell 2 in FIG. 3 serves as a displaysurface DP.

That is, light emitted from the backlight 5 passes through the backlight5, the rear liquid crystal cell 3, the diffusion layer 4, and the frontliquid crystal cell 2 in order and reaches the viewer.

The front liquid crystal cell 2 and the rear liquid crystal cell 3 areseparate from each other. In addition, the diffusion layer 4 is arrangedin a gap 8 between the front liquid crystal cell 2 and the rear liquidcrystal cell 3.

The front liquid crystal cell 2 includes substrates 122 and 124, aliquid crystal layer 123, and polarizing plates 121 and 125.

The substrates 122 and 124 are made from, for example, a glasssubstrate, and are arranged to face each other.

A pixel electrode is formed for each subpixel 26 on a surface of thesubstrate 122, the surface facing the substrate 124, and a pixel voltageis applied thereto by the above-described horizontal driving unit 23.

An electrode common to each subpixel 26 is formed on a surface of thesubstrate 124, the surface facing the substrate 122. Further, a colorfilter and a black matrix are formed on the substrate 124.

The liquid crystal layer 123 is sealed between the substrate 122 and thesubstrate 124, and changes light transmittance in accordance with thepixel voltage applied to the pixel electrode of the substrate 122.

The polarizing plate 121 is attached to the light incident side of thesubstrate 122, and the polarizing plate 125 is attached to the lightemitting side of the substrate 124. A transmission axis of thepolarizing plate 121 and a transmission axis of the polarizing plate 125intersect each other.

The rear liquid crystal cell 3 includes substrates 132 and 134, a liquidcrystal layer 133, and polarizing plates 131 and 135.

The substrates 132 and 134 are made from, for example, a glasssubstrate, and are arranged to face each other.

A pixel electrode is formed for each subpixel 36 on a surface of thesubstrate 132, the surface facing the substrate 134, and a pixel voltageis applied thereto by the above-described horizontal driving unit 33.

Note that there is a structure including no subpixel as described above.In that case, a pixel electrode is formed for each pixel 35.

An electrode common to each subpixel 36 is formed on a surface of thesubstrate 134, the surface facing the substrate 132. Further, a blackmatrix is formed on the substrate 134. In addition, unlike the substrate124 of the front liquid crystal cell 2, no color filter is formed on thesubstrate 134.

The liquid crystal layer 133 is sealed between the substrate 132 and thesubstrate 134, and changes light transmittance in accordance with thepixel voltage applied to the pixel electrode of the substrate 132.

The polarizing plate 131 is attached to the light incident side of thesubstrate 132, and the polarizing plate 135 is attached to the lightemitting side of the substrate 134. A transmission axis of thepolarizing plate 131 and a transmission axis of the polarizing plate 135intersect each other.

The diffusion layer 4 diffuses light incident from the rear liquidcrystal cell 3 side. The diffusion layer 4 can be, for example, adiffusion film in which beads are randomly scattered on a resin film orin the resin film.

The diffusion layer 4 is for reducing moire in a display image. That is,moire may be generated in a display image in the liquid crystal displaypanel 1 because the front liquid crystal cell 2 and the rear liquidcrystal cell 3 that are two liquid crystal display panels are arrangedto be superimposed on each other. Therefore, the diffusion layer 4 isarranged between the front liquid crystal cell 2 and the rear liquidcrystal cell 3 in the liquid crystal display panel 1, thereby reducingmoire and suppressing deterioration in image quality.

The diffusion layer 4 may be arranged at any position in the gap 8, butis desirably arranged on the side close to the front liquid crystal cell2 as illustrated in FIG. 3 . That is, in an inter-panel distance d, adistance d1 between the diffusion layer 4 and the front liquid crystalcell 2 is desirably smaller than a distance d2 between the diffusionlayer 4 and the rear liquid crystal cell 3 (d1<d2).

In this case, a transparent material layer may be formed between thediffusion layer 4 and the front liquid crystal cell 2 and/or between thediffusion layer 4 and the rear liquid crystal cell 3.

Further, the diffusion layer 4 is more desirably arranged to be adjacentto the front liquid crystal cell 2 (d1=0). This is because moire can besuppressed more effectively and sharpness can be improved as thediffusion layer 4 is closer to the front liquid crystal cell 2.

Moire can be suppressed more effectively as a degree of diffusion (hazevalue) of the diffusion layer 4 is higher. For example, in a case wherethe haze value is 90% or more, it is possible to improve a degree offreedom in designing the inter-panel distance d for obtaining desiredimage quality. However, when the haze value is increased, reduction inluminance is concerned. Therefore, it is desirable to reduce resolutionof the rear liquid crystal cell 3 and eliminate the color filter fromthe rear liquid crystal cell 3.

Further, even in a case where the diffusion layer 4 has a low hazevalue, it is possible to obtain desirable image quality by, for example,arranging the diffusion layer 4 close to the front liquid crystal cell2.

The backlight 5 includes not only a light emitting array 42 but also adiffusion plate 141. The diffusion plate 141 diffuses light emitted fromthe light emitting array 42.

The light emitting array 42 is formed by arraying LEDs, for example.

2. Configuration of Dual Cell Image Processing Unit

A configuration of the dual cell image processing unit 12 will bedescribed with reference to FIGS. 4 and 5 .

FIG. 4 is a block diagram of the dual cell image processing unit 12, andFIG. 5 is a block diagram illustrating inside of a rear image generationunit 51 of FIG. 4 in detail.

As illustrated in FIG. 4 , the dual cell image processing unit 12includes an RGB input unit 50, the rear image generation unit 51, afront image generation unit 52, a light amount correction unit 53, panelgamma processing units 54 and 57, adjustment units 55 and 58, a rearoutput unit 56, and a front output unit 59.

The image signal Sig_in from the display image processing unit 11 isinput to the RGB input unit 50, and is supplied from the RGB input unit50 to the rear image generation unit 51.

The image signal Sig_in is, for example, a signal of 1024 gradations of10 bits for each of R, G, and B. As a matter of course, the signal of1024 gradations (10 bits) is merely an example for convenience ofdescription, and a signal having a smaller or larger number ofgradations (bit length) may be used.

The rear image generation unit 51 generates the rear image signal Sig_REin processing described later. The rear image signal Sig_RE is, forexample, a signal of 1024 gradations (10 bits: 0 to 1023) of white (W).

The rear image signal Sig_RE is subjected to gamma processingcorresponding to the rear liquid crystal cell 3 in the panel gammaprocessing unit 54, and is then subjected to necessary adjustmentprocessing in the adjustment unit 55. Then, the rear image signal Sig_REis subjected to delay adjustment processing, parallelizationcorresponding to the three subpixels 36, and the like in the rear outputunit 56 and is then supplied to the rear liquid crystal cell drivingunit 30.

Note that, in a case where no subpixel is formed in the rear liquidcrystal cell 3 as described above, the rear image signal Sig_RE isoutput corresponding to the front image signal Sig_FR at three timingscorresponding to the three subpixels 26R, 26G, and 26B in the frontliquid crystal cell 2.

The rear image signal Sig_RE generated in the rear image generation unit51 is also supplied to the light amount correction unit 53. The lightamount correction unit 53 multiplies the rear image signal Sig_RE by alight amount correction coefficient kLC for correcting a light amountcomponent incident on the front liquid crystal cell 2, and outputs themultiplied signal to the front image generation unit 52.

The light amount correction coefficient kLC is, for example, a fixedvalue. However, the light amount correction coefficient kLC may be avariable value. For example, the light amount correction coefficient kLCmay be adaptively calculated in accordance with an image.

The image signal Sig_in is supplied to the front image generation unit52. The front image generation unit 52 includes a division unit 52 a asillustrated in FIG. 5 , and generates a front image signal Sig_FR bydividing the input image signal Sig_in by the rear image signal Sig_RE.

In a case of the dual-cell liquid crystal display panel 1, an image inwhich an image in the rear liquid crystal cell 3 and an image in thefront liquid crystal cell 2 are combined is visually recognized as adisplay image. That is, an image in which the luminance in the rearliquid crystal cell 3 and the luminance in the front liquid crystal cell2 are multiplied is displayed. Therefore, when the luminance of thefront image signal Sig_FR is divided by the luminance of the rear imagesignal Sig_RE, it is possible to perform display according to theluminance of the original image signal Sig_in for each pixel. For such areason, the front image signal Sig_FR is generated by dividing the imagesignal Sig_in by the rear image signal Sig_RE.

However, in actuality, a light amount difference is generated betweenlight emitted from the rear liquid crystal cell 3 and light incident onthe front liquid crystal cell 2, and thus, to be exact, it is necessaryto perform correction instead of simple division. Thus, the rear imagesignal Sig_RE is corrected in the light amount correction unit 53(Sig_RE·kLC).

Therefore, the front image generation unit 52 (division unit 52 a)calculates the front image signal Sig_FR as follows:

Sig_FR = Sig_in/(Sig_RE ⋅ kLC).

Herein, the image signal Sig_in is a signal including gradation valuesSig_in(R), Sig_in(G), and Sig_in(B) of R, G, and B. Thus, morespecifically, gradation values Sig_FR(R), Sig_FR(G), and Sig_FR(B) of R,G, and B of the front image signal Sig_FR are generated as follows:

Sig_FR(R) = Sig_in(R)/(Sig_RE ⋅ kLC)Sig_FR(G) = Sig_in(G)/(Sig_RE ⋅ kLC)Sig_FR(B) = Sig_in(B)/(Sig_RE ⋅ kLC).

Each of the gradation values Sig_FR(R), Sig_FR(G), and Sig_FR(B) of R,G, and B is, for example, a signal of 1024 gradations (0 to 1023) of 10bits.

Note that, in a case where the liquid crystal display panel 1 is lessnecessary to perform light amount correction or in other cases,Sig_FR=Sig_in/Sig_RE may be calculated, without providing the lightamount correction coefficient kLC.

The front image signal Sig_FR generated in the front image generationunit 52 is subjected to gamma processing corresponding to the frontliquid crystal cell 2 in the panel gamma processing unit 57 of FIG. 4 ,and is then subjected to necessary adjustment processing in theadjustment unit 58. Then, the front image signal Sig_FR is subjected toparallelization and the like corresponding to the three subpixels 26R,26G, and 26B in the front output unit 59 and is then supplied to thefront liquid crystal cell driving unit 20.

A configuration inside the rear image generation unit 51 will bedescribed with reference to FIG. 5 .

The rear image generation unit 51 converts the image signal Sig_in thatis the input color image signal into a grayscale signal (black-and-whiteimage signal) Gr in a grayscale conversion unit 70.

Grayscale conversion is performed by using coefficients kR, kG, and kBas follows:

Gr = kR ⋅ Sig_in(R) + kG ⋅ Sig_in(G) + kB ⋅ Sig_in(B).

That is, the coefficient kR for R, the coefficient kG for G, and thecoefficient kB for B are multiplied by the respective gradation valuesSig_in(R), Sig_in(G), and Sig_in(B) of R, G, and B included in the imagesignal Sig_in. Then, the resultant values are added to become thegrayscale signal Gr. The grayscale signal Gr has a gradation value (0 to1023) as white (W).

Note that a method of setting the coefficients kR, kG, and kB will bedescribed later.

Such a grayscale signal Gr is supplied to a gradation conversion unit 72and is subjected to gradation value conversion.

The gradation conversion unit 72 includes a lookup table (LUT) 73 and agamma conversion unit 74. The gamma conversion unit 74 refers to the LUT73 by using the gradation value of the input grayscale signal Gr as anLUT input signal LUT_in, thereby obtaining a corresponding outputgradation value (LUT output signal LUT_out). Then, the output gradationvalue (LUT output signal LUT_out) is supplied to a combination unit 78.

In the gradation conversion unit 72, the grayscale signal Gr issubjected to the gradation value conversion so as to become a signal forperforming gradation expression in a gradation region set as a gradationrange in which the color gamut greatly changes in a case where the frontliquid crystal cell 2 is driven by the color image signal.

Although the setting of the LUT 73 will be described later, in thisembodiment, the color gamut expansion in the low gradation region isrealized particularly by the coefficients kR, kG, and kB used in thegrayscale conversion unit 70 and the gradation value conversion in theLUT 73.

Note that, in the rear image generation unit 51, a gradation maintainingunit 75 is provided for gradation reproduction at the time of a highgradation.

The gradation maintaining unit 75 detects a maximum value among thegradation values Sig_in(R), Sig_in(G), and Sig_in(B) of R, G, and B,respectively, in the image signal Sig_in, and performs predeterminedprocessing on the detected maximum value, thereby generating a signal M.The signal M is supplied to the combination unit 78.

In the combination unit 78, regarding the image signal Sig_in at acertain point in time, a gradation value of the signal M and thegradation value of the LUT output signal LUT_out are supplied at thesame timing, and the larger gradation value therebetween is selected andserves as the gradation value of the rear image signal Sig_RE.

Output (rear image signal Sig_RE) of the combination unit 78 isprocessed by a spatial processing unit 79 and is then output from therear image generation unit 51.

From this, roughly speaking, in a case where the image signal Sig_in fora pixel in the high gradation region is input, the gradation value ofthe signal M tends to be reflected in the rear image signal Sig_RE, and,in a case where the image signal Sig_in for a pixel in the low gradationregion is input, the gradation value of the LUT output signal LUT_outtends to be reflected in the rear image signal Sig_RE. However, thiscannot be unconditionally said because the output gradation value of theLUT 73 does not simply correspond to the gradation of the image signalSig_in.

The signal M is used because there is a possibility that, for example,in a case where a signal of 1023 gradations of a single color R is inputor in other cases, the signal is not returned to 1023 gradations due tothe grayscale conversion and the conversion using the LUT 73. Thisdepends on settings of the coefficients kR, kG, and kB and the LUT 73.

For such a case, in order to maintain a gradation performance, thesignal M based on the maximum value is used in some cases.

3. Improvement in Color Gamut in Embodiment

Processing of the dual cell image processing unit 12 having the aboveconfiguration, in particular, improvement in the color gamut in the lowgradation region realized by processing of the rear image generationunit 51 will be described.

As described above, one liquid crystal cell has a small amount of lightleakage due to liquid crystal characteristics, and therefore cannotreproduce perfect black.

FIG. 6 illustrates a measured value wMD of luminance of white (W) of oneliquid crystal cell and a curve of gamma of 2.2, where a horizontal axisshows an input gradation and a vertical axis shows luminance([cd/m^(2]).)

Note that “W” herein is a case where an R gradation, a G gradation, anda B gradation are the same. For example, the measured value of theluminance obtained when the R gradation value=the G gradation value=theB gradation value=“10” is a luminance value obtained in a case where aninput gradation value is “10”.

The gradation on the horizontal axis shows a range of 0 to 1000 out of1024 gradations of 0 to 1023 on the assumption of, for example, 10-bitdata.

In terms of performance, the curve of the measured value wMD preferablymatches the gamma of 2.2. However, the luminance is not sufficientlyreduced in the low gradation region as illustrated in FIG. 6 . Thisindicates that lower luminance cannot be obtained due to light leakage.

In addition, original gradation cannot be output in the low gradation asdescribed above, which results in a decrease in contrast.

Further, a change in chromaticity is increased (the color gamut isdecreased) at the time of a low gradation due to this light leakage, andcharacteristics of R, G, and B differ from each other in some cases asillustrated in FIG. 7 .

FIG. 7 illustrates a change in chromaticity with respect to a gradationat the time of outputting each of the single colors R, G, and B by usinga value of each of x, y, u′, v′, and Δu'v′ as axes of a chromaticitydiagram. A horizontal axis shows an input gradation, which indicates arange of 0 to 1000 out of 0 to 1023. A vertical axis shows chromaticity.

Those diagrams show that the change in chromaticity is increased in thelow gradation region due to an influence of light leakage (in actuality,the color looks lighter).

Further, the diagrams also show that such a change in chromaticity isnot uniform in R, G, and B.

The problem caused by using one liquid crystal cell as described aboveis solved by adopting the dual cell configuration and using theconfigurations of FIGS. 4 and 5 described above in this embodiment.

In particular, a display gradation of the rear liquid crystal cell 3 onthe backlight side (black-and-white side) is adjusted by the LUTconversion so as to use only a gradation range in which a change in thecolor gamut of the front liquid crystal cell 2 serving as a color cellis small.

Specifically, a method of improving the gradation performance colorgamut by using the grayscale conversion performed by the grayscaleconversion unit 70 and the LUT gradation conversion performed by thegamma conversion unit 72 and the LUT 73 will be described as imageprocessing for the rear liquid crystal cell 3 as described in (1) to (5)below.

(1) Setting of Lower Limit Value Target by Checking Single ColorGradation Chromaticity of Front Liquid Crystal Cell 2

The coefficients kR, kB, and kG of the respective colors used in thegrayscale conversion unit 70 are set.

For this, first, output chromaticity characteristics with respect to theinput gradation of each single color in one cell (the front liquidcrystal cell 2 in this embodiment) are checked. The characteristics aredifferent between display panels, and thus it is desirable to checkcharacteristics by using an actual apparatus.

Then, based on a result obtained as characteristics, an input gradationrange (lower limit value target) in which the change in chromaticity issmall is set.

FIG. 8 , as well as FIG. 7 , illustrates the change in chromaticity withrespect to the gradation at the time of outputting each of the singlecolors R, G, and B.

As the lower limit value target, a lower limit of the input gradationrange in which the change in chromaticity is small is determined. Forexample, in each single color, the lower limit value target is set to agradation range up to Δu′v′=0.033 (10 JND).

In this case, the lower limit value target is provisionally set asfollows:

R_mingradation = 239/1023G_mingradation = 127/1023B_mingradation = 255/1023.

(2) Provisional Setting of Grayscale Conversion Parameters for RearLiquid Crystal Cell 3

Reciprocals of the lower limit values of the respective gradations setin the above (1) are defined as parameter coefficients kR, kG, and kB.Note that the reciprocals are divided by a denominator so that kR, kG,and kB become “1” in total.

kR = 1/R_mingradation/(1/R_mingradation + 1/G_mingradation + 1/B_mingradation)kG = 1/G_mingradation/(1/R_mingradation + 1/G_mingradation + 1/B_mingradation)kB = 1/B_mingradation/(1/R_mingradation + 1/G_mingradation + 1/B_mingradation)

When the above provisionally set R min gradation, G min gradation, and Bmin gradation are substituted into those expressions,

kR = 0.26kG = 0.49kB = 0.25

are obtained. In this way, the coefficients kR, kG, and kB areprovisionally set.

As a tendency of the coefficients, a color having a larger target lowerlimit value (a color that greatly changes in the low gradation region)has a smaller coefficient value (a lower rear output gradation).

(3) Setting of Target Input Gradation

It is determined up to which gradation value of the color image signalSig_in serving as the input gradation the color gamut is secured.

When luminance is decreased, human beings hardly perceive color due tocolor vision characteristics of eyes (called scotopic vision to mesopicvision), which is not only for the liquid crystal panel.

Therefore, even if a color is reproduced in a low gradation equal to orlower than a gradation in which the color cannot be perceived, asufficient effect of improvement in the image quality is hardlyobtained. In other words, it is considered that an influence on theimage quality is extremely small even if the color gamut is decreased inextremely low gradations.

Further, an increase in a gradation region for securing the color gamutis traded off against a problem of the image quality such as a doubleimage. The double image is a phenomenon in which a viewer perceives adisplay image as if the display image is doubled due to a shadow of therear liquid crystal cell 3.

Therefore, a sensory evaluation of an actual apparatus by visualobservation is performed to determine up to which input gradation asingle color gamut is secured.

For the sensory evaluation, there is used the liquid crystal displaypanel 1 that can secure a sufficient color gamut even at the time of thelow gradation with the luminance of about 1000 [cd/m²] when the inputgradation is set to the maximum value with a panel gamma of 2.2.

The maximum value of the input gradation is a case where the Rgradation=1023/1023, the G gradation=1023/1023, and the Bgradation=1023/1023 are satisfied.

The sensory evaluation shows that a significant difference in the colorgamut is reduced in a case where the input gradations of the singlecolors of R, G, and B are equal to or lower than 80/1023.

FIG. 9 illustrates gamma of each color obtained in a case where theinput gradation is set to the maximum value with the panel gamma of 2.2.

In a range of the input gradation of 80/1023 or less and the luminanceof 3 [cd/m²] or less, which is surrounded by a broken line, the humaneyes have a low sensitivity to color and therefore hardly perceive adifference even if the chromaticity changes. That is, an effect of awide color gamut is hardly obtained in the input gradation of 80/1023 orless.

Thus, the input gradation of 80/1023 is provisionally set as the targetinput gradation.

(4) Setting of LUT for Gamma Conversion of Rear Liquid Crystal Cell 3

An LUT value (curve) is determined from the above provisionally setvalues in the above (1), (2), and (3) so as to satisfy conditions. Agradation relationship (chromaticity characteristics) between the frontliquid crystal cell 2 and the rear liquid crystal cell 3 other than thetarget gradations changes depending on how the curve is formed, but thisembodiment is limited to a region of the target gradations herein.

Herein, there will be described an example in which an LUT conversionsetting is performed so as to obtain a lower limit value targetgradation of the front liquid crystal cell 2 at the time of thesingle-color target input gradation (80/1023) of the above (3). [0066]

The following expressions are as follows.

The single-color target input gradation of the single color R, i.e.,input of an R value=80/1023, a G value=0/1023, and a B value=0/1023 isexpressed as “Sig_in (80, 0, 0)”.

The single-color target input gradation of the single color G, i.e.,input of the R value=0/1023, the G value=80/1023, and the B value=0/1023is expressed as “Sig_in (0, 80, 0)”.

The single-color target input gradation of the single color B, i.e.,input of the R value=0/1023, the G value=0/1023, and the B value=80/1023is expressed as “Sig_in (0, 0, 80)”.

The grayscale signal Gr from the grayscale conversion unit 70 isexpressed as “Gr(*)” (“*” denotes a value between 0 and 1023 indicatinga gradation value of */1023).

The gradation value of the rear image signal Sig_RE is expressed asSig_RE (*).

The gradation values of the LUT input signal LUT_in and the LUT outputsignal LUT_out are expressed as LUT_in (*) and LUT_out (*).

The gradation value of the front image signal Sig_FR is expressed asSig_FR (*, *, *). “(*, *, *)” indicates (R value, G value, B value).

The input gradation value Sig_in(80, 0, 0) of the single color R issubjected to grayscale conversion by using the coefficients kR, kG, andkB that are provisionally set as described above, and thus becomes agrayscale signal Gray(21) having the gradation value of 21/1023.

Herein, the lower limit value target is provisionally set to R_mingradation=239/1023.

In order to obtain the front image signal Sig_FR=Sig_F(239, 0, 0), it isonly necessary to obtain the rear image signal Sig_RE so that a divisionresult of the R value in the division unit 52 a is “239”. Therefore, itis only necessary to obtain the rear image signal Sig_RE=Sig_RE(342)(herein, the coefficient of the light amount correction unit 53 isignored for simplification of the description).

Therefore, it is only necessary to convert the grayscale signal Gray(21)into the rear image signal Sig_RE(342). Thus, LUT_in(21)→LUT_out(342)needs to be satisfied (the gradation value of 21/1023 needs to beconverted into the gradation value of 342/1023) as a condition for thesingle color R.

The input gradation value Sig_in(0, 80, 0) of the single color G issubjected to grayscale conversion by using the provisionally setcoefficients kR, kG, and kB, and thus becomes a grayscale signalGray(39).

The lower limit value target is provisionally set to G_mingradation=127/1023.

In order to obtain the front image signal Sig_FR=Sig_F(0, 127, 0), it isonly necessary to obtain the rear image signal Sig_RE(644) so that adivision result of the G value in the division unit 52 a is “127”.

Therefore, it is only necessary to convert the grayscale signal Gray(39)into the rear image signal Sig_RE(644), and thus LUT in(39)→LUT out(644)needs to be satisfied (the gradation value of 39/1023 needs to beconverted into the gradation value of 644/1023) as a condition for thesingle color G.

The input gradation value Sig_in(0, 0, 80) of the single color B issubjected to grayscale conversion by using the provisionally setcoefficients kR, kG, and kB, and thus becomes a grayscale signalGray(20).

The lower limit value target is provisionally set to B_mingradation=255/1023.

In order to obtain the front image signal Sig_FR=Sig_F(0, 0, 255), it isonly necessary to obtain the rear image signal Sig_RE (321) so that adivision result of the B value in the division unit 52 a is “255”.

Therefore, it is only necessary to convert the grayscale signal Gray(20)into the rear image signal Sig_RE(321), and thus LUT_in(20)→LUT_out(321)needs to be satisfied (the gradation value of 20/1023 needs to beconverted into the gradation value of 321/1023) as a condition for thesingle color B.

From the above, it is only necessary to design (provisionally set) theLUT 73 so that conversion is performed to satisfy

LUT_in(21)→LUT_out(342) . . . R single-color condition

LUT_in(39)→LUT_out(644) . . . G single-color condition

LUT_in(20)→LUT_out(321) . . . B single-color condition.

FIG. 10B illustrates a conversion curve of the input gradation(horizontal axis)—the output gradation (vertical axis) in theprovisionally set LUT 73, and FIG. 10A is an enlarged view of a part ofFIG. 10B (a part surrounded by a broken line).

In FIG. 10A, a point Rp, a point Gp, and a point Bp are shown oncoordinates. The point Rp shows coordinates of LUT_in(21)→LUT_out(342)as the R single-color condition. Similarly, the points Gp and Bp showcoordinates indicating LUT input/output values of the above Gsingle-color condition and B single-color condition, respectively.

An approximate curve including the points Rp, Gp, and Bp is aprovisional setting example of the LUT 73.

When the setting is performed by using a function of an approximatevalue, the above conditions are not always strictly satisfied, but thesetting is effective in a case where the conditions are roughlysatisfied.

Such input/output characteristics of the LUT 73 according to thisembodiment do not define the shape of the conversion curve but arereference for determining a front and rear gradation relationship at thetime of the low gradation. Thus, when the input/output characteristicsof the LUT 73 are set to satisfy the above conditions, a target frontgradation and chromaticity are obtained at the time of the lowgradation, and the color gamut is secured.

Further, the characteristics serve as an element that affects not onlythe color gamut but also various characteristics such as a double image,a flare, and a contrast that are problems in image quality of two cells.

Note that, in the conversion curve of the LUT 73 in FIG. 10B, the outputgradation value=1023/1023 is set in a region of approximately high inputgradation values (for example, a region of 400 or more). This isbecause, in the high gradation region, gradation is expressed only bythe front liquid crystal cell 2, whereas the rear liquid crystal cell 3is opened.

As described above, in one liquid crystal panel, light leakage and achange in chromaticity may be increased in the low gradation region. Inthis embodiment, the light leakage and change in chromaticity are solvedby the front liquid crystal cell 2 and the rear liquid crystal cell 3.In this case, the rear liquid crystal cell 3 serves as a complement forthe low gradation region affected by light leakage and a change inchromaticity in the front liquid crystal cell 2. Thus, it is consideredthat an output gamma characteristic of the rear image signal Sig_REpreferably adjusts a gradation only in the low gradation region and ispreferably transmitted (white) in the high gradation region in which noproblem arises by using only the front liquid crystal cell 2. Therefore,as illustrated in FIG. 10B, LUT conversion is performed so as to openthe rear liquid crystal cell 3 in the high gradation region.

(5) Confirmation of Output Gradation in Dual Cell Configuration

FIGS. 11A-11C illustrate output gradation relationships between twoliquid crystal cells under the above provisional setting conditions (1)to (4).

FIG. 11A illustrates the input/output characteristics of the rear liquidcrystal cell 3, where a horizontal axis shows the input gradation valueand a vertical axis shows the output gradation value. Herein, R, G, B,and white (W) are shown.

FIG. 11B illustrates the input/output characteristics of the frontliquid crystal cell 2, where a horizontal axis shows the input gradationvalue and a vertical axis shows the output gradation value. Herein, R,G, and B are shown. FIG. 11C is an enlarged view of a low gradationregion surrounded by a broken line in FIG. 11B.

Output gradation curves of the rear liquid crystal cell 3 and the frontliquid crystal cell 2 in FIGS. 11A and 11B are changed by the aboveconversion curve of the LUT 73 in FIG. 10B. As can be seen from theenlarged view of FIG. 11C, the front image signal Sig_FR becomes outputnear the min gradation set for each single color in the input targetgradation (80/1023), and becomes output equal to or larger than a minsetting gradation in the input gradation equal to or larger than theinput target gradation.

For example, in a case where the input target gradation of the singlecolor R is (80/1023), the output gradation is near R_mingradation=239/1023.

In a case where the input target gradation of the single color G is(80/1023), the output gradation is near G_min gradation=127/1023.

In a case where the input target gradation of the single color B is(80/1023), the output gradation is near B_min gradation=255/1023.

Therefore, the change in chromaticity is suppressed within the setrange, and the target color gamut is secured.

Meanwhile, as illustrated in FIG. 11A, the gradation in the lowgradation region is expressed by the rear liquid crystal cell 3.

That is, as a concept, as illustrated in FIG. 8 , the lower limit valuetarget of the single color gradation chromaticity is set so that, in thefront liquid crystal cell 2, color expression is suppressed in the lowgradation region in which the chromaticity greatly changes in each ofcolors R, G, and B and a degree of the change is different therebetween.In addition, output of (near) the lower limit value target gradation isobtained in the front liquid crystal cell 2 in a case of the inputtarget gradation of each of the single colors R, G, and B. The gradationexpression in the low gradation region is performed by the rear liquidcrystal cell 3.

According to the above consideration, the coefficients kR, kG, and kB ofthe grayscale conversion and the characteristics of the LUT 73 are setas described above in order to use a gradation range in which the changein chromaticity is small in the front liquid crystal cell 2. Thissuppresses a change in the color gamut.

As described above, this embodiment sets the coefficients kR, kG, and kBused for grayscale conversion processing in the grayscale conversionunit 70 and performs conversion using the LUT 73, thereby reducing achange in the color gamut caused by a gradation. This results in anincrease in the color gamut in lower gradations, thereby improving theimage quality.

FIGS. 12A and 12B illustrate examples of an improvement effect.

FIG. 12A illustrates DCI coverage in a comparative example and thisembodiment. The comparative example is a case of a liquid crystaldisplay panel having one liquid crystal cell.

Herein, coverage at luminance levels of 100 IRE, 10 IRE, and 5 IRE isshown.

FIG. 12B illustrates color gamuts Cg1, Cg2, Cg3, Cg4, and Cg5 in a u′v′chromaticity diagram. The color gamut Cg1 shows a case of 100 IRE, thecolor gamut Cg2 shows a case of the comparative example (10IRE), thecolor gamut Cg3 shows a case of the comparative example (5IRE), thecolor gamut Cg4 shows a case of the embodiment (10IRE), and the colorgamut Cg5 shows a case of the embodiment (5IRE).

From those diagrams, it is possible to confirm a significant improvementin the color gamut in the low gradation in this embodiment.

4. Conclusion and Modification Example

In the above embodiment, the following effects can be obtained.

The image processing device (dual cell image processing unit 12) of theembodiment includes the grayscale conversion unit 70 (black-and-whiteconversion unit) that converts an image signal Sig_in serving as a colorimage signal for the liquid crystal display panel 1 in which a displayimage is generated by light passing through the rear liquid crystal cell3 and the front liquid crystal cell 2 into a grayscale signal Gr(black-and-white image signal) by using the predetermined coefficientskR, kG, and kB. Further, the dual cell image processing unit 12 includesthe gradation value conversion unit 72 that performs gradation valueconversion so that the grayscale signal Gr obtained by the grayscaleconversion unit 70 serves as a signal for performing gradationexpression in a gradation region that is set as a gradation range inwhich the color gamut greatly changes in a case where the front liquidcrystal cell 2 is driven by the color image signal, thereby generating arear image signal Sig_RE serving as the black-and-white image signal forthe rear liquid crystal cell 3. Further, the dual cell image processingunit 12 includes the front image generation unit 52 that performsarithmetic processing using the rear image signal Sig_RE on the imagesignal Sig_in, thereby generating a front image signal Sig_FR serving asthe color image signal for the front liquid crystal cell 2.

With this configuration, for example, when gradation expression can besufficiently performed in the low gradation region by the rear imagesignal Sig_RE, it is possible to achieve a state in which, regarding thefront image signal Sig_FR, the gradation value in the low gradationregion in which the color gamut greatly changes does not need tocontribute the gradation expression. In other words, even if, in thefront image signal Sig_FR, the gradation value in the low gradationregion in which the color gamut greatly changes is not used, it ispossible to obtain a display image in which gradation expression issufficiently performed also in the low gradation region in the liquidcrystal display panel 1. Therefore, in the embodiment, regarding thefront image signal Sig_FR, the gradation region in which the color gamutis narrow is not used. This makes it possible to maintain the colorgamut even in the low gradation region. That is, it is possible toimprove the image quality because a change in the color gamut caused bya gradation is reduced and the color gamut is increased in lowergradations.

Further, a balance of a double image that is traded off against anecessary gradation color gamut can be maintained by performing thegradation value conversion using the LUT 73 and setting the coefficientskR, kG, and kB for the grayscale conversion in the grayscale conversionunit 70.

The grayscale conversion unit 70 according to the embodiment convertsthe color image signal into the black-and-white image signal byperforming calculation using a coefficient calculated by using a lowerlimit value of a gradation value set for each color included in thecolor image signal.

That is, first, the lower limit values (R min gradation, G_mingradation, and B_min gradation) of the gradation value of respectivecolors (for example, R, G, and B) included in the color image signal areset. That is, each lower limit values is a lower limit value of agradation value of the front image signal Sig_FR. By using the lowerlimit values, the coefficients (kR, kG, and kB) for converting the colorimage into the black-and-white image are set. The black-and-whiteconversion unit converts the color image signal Sig_in into theblack-and-white image signal (grayscale signal Gr) by calculation usingthe coefficients.

When the color image signal is converted into the black-and-white imagesignal in this way, image processing reflecting the lower limit valuesof the gradation values of the front image signal Sig_FR is realized.That is, in a case where a gradation region in which a change in thecolor gamut is small is used as the front image signal Sig_FR, it ispossible to set the lower limit of the gradation region. When the lowerlimit values in the gradation region are appropriately selected, aneffect of reducing the change in the color gamut is appropriatelyexhibited.

In particular, the embodiment shows an example in which the grayscaleconversion unit 70 converts a color image signal into a black-and-whiteimage signal by performing calculation using the coefficients kR, kG,and kB of R, G, and B, respectively, each of the coefficients beingcalculated by using the lower limit value of the gradation value set foreach of R, G, and B.

That is, the lower limit values (R_min gradation, G_min gradation, andB_min gradation) of the gradation values of R, G, and B included in thecolor image signal are separately set. By using the lower limit values,the coefficients (kR, kG, and kB) for the gradation values of R, G, andB for converting the color image into the black-and-white image are set.The black-and-white conversion unit converts the color image signalSig_in into the black-and-white image signal (grayscale signal Gr) bycalculation using the coefficients.

The colors R, G, and B are different from each other in a degree ofchange in chromaticity in the low gradation region. Therefore, it isdesirable to individually set the lower limit values of the gradationvalues of R, G, and B. When the coefficients for the respectivegradation values of R, G, and B are set on the basis of the lower limitvalues of the respective gradation values of R, G, and B and grayscaleconversion is performed by using the coefficients, it is possible toperform image processing in consideration of a difference in the changein the chromaticity between the colors. That is, it is possible to setprocessing in accordance with an actual value of a color gamut gradationrange of each of R, G, and B in the front liquid crystal cell 3.

Further, the lower limit value described in the embodiment is agradation value of each of R, G, and B at which the chromaticity changesto a predetermined value in the low gradation region.

In a case of the embodiment, the lower limit values (R_min gradation,G_min gradation, and B_min gradation) of the gradation values of R, G,and B are gradation values that satisfy Δu′v′=0.033 (10 JND). When thelower limit values of R, G, and B are set by using such a reference, itis possible to set coefficients in accordance with the change in thechromaticity in the low gradation region in each of R, G, and B. Thismakes it possible to improve accuracy of image processing so that thechange in the chromaticity is reduced in the low gradation region.

In the embodiment, the gradation value conversion unit 72 performs thegradation value conversion by using the LUT 73 in which an output valueis stored for an input value.

With a design of the LUT 73, it is possible to appropriately set the LUToutput signal LUT_out for the LUT input signal LUT_in with a high degreeof freedom. This makes it possible to accurately and easily realize thegradation value conversion in which the rear image signal serves as asignal for performing gradation expression in a gradation region set asa gradation range in which the color gamut greatly changes in the frontliquid crystal cell.

In the embodiment, the lower limit value of the gradation value of eachcolor included in the color image signal and a target input gradation(for example, 80/1023) of the gradation value of the color image signalare set, and the LUT 73 is set so that, when the color image signalhaving the target input gradation of a single color is input, the frontimage signal Sig_FR becomes the lower limit value of the single color.

When luminance is decreased, human beings hardly perceive color due tocolor vision characteristics of eyes. Therefore, in a case of anextremely low gradation, the image processing of the embodiment is notuseful in some cases. Therefore, the target input gradation is set asthe minimum gradation at which color can be perceived, and the LUT 73 isset so as to have the lower limit value in a case of the target inputgradation. This makes it possible to improve the color gamut in asignificant gradation region.

Further, the target input gradation is set as a maximum gradation valuein a low gradation region in which human eyes hardly perceive a changein chromaticity.

For example, when a gradation in which human eyes hardly perceive achange in the color gamut is set as the target input gradation bysensory evaluation, it is possible to prevent unnecessary processingfrom being actually performed in a gradation region in which color gamutexpansion is not useful.

In the embodiment, the front image generation unit 52 generates a frontimage signal by dividing the image signal Sig_in by the rear imagesignal Sig_RE.

That is, R, G, and B gradation values of the front image signal areobtained by, for example, dividing the gradation values of R, G, and Bof the image signal Sig_in serving as the color image signal by therespective gradation values of the rear image signal Sig_RE.

By dividing the color image signal by the rear image signal, it ispossible to obtain an appropriate gradation as an image in which therear liquid crystal cell 3 and the front liquid crystal cell 2 aresuperimposed.

The embodiment shows an example in which the dual cell image processingunit 12 includes the light amount correction unit 53 that multiplies therear image signal Sig_RE by the correction coefficient kLC correspondingto a light amount component incident on the front liquid crystal cell 2,and the front image generation unit 52 generates the front image signalSig_FR by dividing the signal Sig_in by the rear image signal Sig_REmultiplied by the correction coefficient kLC.

It is possible to obtain the front image signal Sig_FR in considerationof the light amount component incident on the front liquid crystal cell2 from the rear liquid crystal cell 3, and obtain an appropriategradation as an image in which the rear liquid crystal cell 3 and thefront liquid crystal cell 2 are superimposed.

The display apparatus 90 according to the embodiment includes: thedual-cell liquid crystal display panel 1 in which a display image isgenerated by light passing through a rear liquid crystal cell and afront liquid crystal cell; and the above-described dual cell imageprocessing unit 12.

The liquid crystal display panel 1 includes the backlight 5, the rearliquid crystal cell 3, the diffusion layer 4, and the front liquidcrystal cell 2 arranged in this order.

In such a dual-liquid-crystal-cell liquid crystal display panel 1, imageprocessing for reducing a change in the color gamut in the low gradationregion is realized by the dual cell image processing unit 12.

Note that the technology disclosed in this embodiment is not limited tothe configuration and the setting method according to the aboveembodiment. The configuration example of the dual cell image processingunit 12, the setting example of the coefficients kR, kG, and kBcoefficients used in the grayscale conversion unit 70, the settingexample of the LUT 73, and the like can be variously modified.

Further, the technology of this embodiment is applied to a liquidcrystal panel in which a color gamut greatly changes in a low gradationregion. However, the technology of this embodiment is applicable notonly to the low gradation region but also to a specific gradation regionin which the color gamut changes.

Note that the effects described in the present specification are merelyillustrative and are not limited. Further, additional effects may beobtained.

Note that the present technology can also be configured as follows.

(1)

An image processing device including:

a black-and-white conversion unit that converts a color image signalinto a black-and-white image signal by using a predeterminedcoefficient, the color image signal being a signal for a display panelin which a display image is generated by light passing through a rearliquid crystal cell and a front liquid crystal cell;

a gradation value conversion unit that performs gradation valueconversion so that the black-and-white image signal obtained in theblack-and-white conversion unit serves as a signal for performinggradation expression in a gradation region set as a gradation range inwhich a color gamut greatly changes in a case where the front liquidcrystal cell is driven by the color image signal, and generates a rearimage signal serving as the black-and-white image signal for the rearliquid crystal cell; and

a front image generation unit that performs arithmetic processing byusing the rear image signal on the color image signal to generate afront image signal serving as the color image signal for the frontliquid crystal cell.

(2)

The image processing device according to (1), in which

the black-and-white conversion unit converts the color image signal intothe black-and-white image signal by performing calculation using acoefficient calculated by using a lower limit value of a gradation valueset for each color included in the color image signal.

(3)

The image processing device according to (2), in which

the color image signal includes gradation values of red, green, andblue; and

the black-and-white conversion unit converts the color image signal intothe black-and-white image signal by performing calculation usingcoefficients of red, green, and blue, each of the coefficients beingcalculated by using a lower limit value of a gradation value set foreach of red, green, and blue.

(4)

The image processing device according to (3), in which

the lower limit value is a gradation value of each of red, green, andblue at which chromaticity changes to a predetermined value in a lowgradation region.

(5)

The image processing device according to any one of (1) to (4), in which

the gradation value conversion unit performs gradation value conversionby using a lookup table in which an output value is stored for an inputvalue.

(6)

The image processing device according to (5), in which:

the lower limit value of the gradation value of each color included inthe color image signal and a target input gradation of the gradationvalue of the color image signal are set; and

the lookup table is set so that, when the color image signal having thetarget input gradation of a single color is input, the front imagesignal becomes the lower limit value of the single color.

(7)

The image processing device according to (6) in which

the target input gradation is set as a maximum gradation value in a lowgradation region in which human eyes hardly perceive a change inchromaticity.

(8)

The image processing device according to any one of (1) to (7), in which

the front image generation unit generates the front image signal bydividing the color image signal by the rear image signal.

(9)

The image processing device according to any one of (1) to (7), furtherincluding

a light amount correction unit that multiplies the rear image signal bya correction coefficient corresponding to a light amount componentincident on the front liquid crystal cell, in which

the front image generation unit generates the front image signal bydividing the color image signal by the rear image signal multiplied bythe correction coefficient in the light amount correction unit.

(10)

A display apparatus including:

a display panel in which a display image is generated by light passingthrough a rear liquid crystal cell and a front liquid crystal cell;

a black-and-white conversion unit that converts a color image signal forthe display panel into a black-and-white image signal by using apredetermined coefficient;

a gradation value conversion unit that performs gradation valueconversion so that the black-and-white image signal obtained in theblack-and-white conversion unit serves as a signal for performinggradation expression in a gradation region set as a gradation range inwhich a color gamut greatly changes in a case where the front liquidcrystal cell is driven by the color image signal, and generates a rearimage signal serving as the black-and-white image signal for the rearliquid crystal cell; and

a front image generation unit that performs arithmetic processing byusing the rear image signal on the color image signal to generate afront image signal serving as the color image signal for the frontliquid crystal cell.

(11)

The display apparatus according to (10), in which

the display panel includes

a light source unit, the rear liquid crystal cell, a diffusion layer,and the front liquid crystal cell arranged in this order.

(12)

An image processing method performed by an image processing device, themethod including:

a black-and-white conversion step of converting a color image signalinto a black-and-white image signal by using a predeterminedcoefficient, the color image signal being a signal for a display panelin which a display image is generated by light passing through a rearliquid crystal cell and a front liquid crystal cell;

a gradation value conversion step of performing gradation valueconversion so that the black-and-white image signal obtained in theblack-and-white conversion step serves as a signal for performinggradation expression in a gradation region set as a gradation range inwhich a color gamut greatly changes in a case where the front liquidcrystal cell is driven by the color image signal, and generating a rearimage signal serving as the black-and-white image signal for the rearliquid crystal cell; and

a front image generation step of performing arithmetic processing byusing the rear image signal on the color image signal to generate afront image signal serving as the color image signal for the frontliquid crystal cell.

REFERENCE SIGNS LIST

-   1 Liquid crystal display panel-   2 Front liquid crystal cell-   3 Rear liquid crystal cell-   4 Diffusion layer-   5 Backlight-   20 Front liquid crystal cell driving unit-   30 Rear liquid crystal cell driving unit-   51 Rear image generation unit-   52 Front image generation unit-   53 Light amount correction unit-   54, 57 Panel gamma processing unit-   55, 58 Adjustment unit-   56 Rear output unit-   57 Front output unit-   70 Grayscale conversion unit-   72 Gradation value conversion unit-   73 LUT-   74 Gamma conversion unit-   75 Gradation maintaining unit-   78 Combination unit-   79 Spatial processing unit-   90 Display apparatus

The invention claimed is:
 1. A display apparatus comprising: a displaypanel in which a display image is generated by light passing through arear liquid crystal cell and a front liquid crystal cell, and includinga diffusion layer arranged between the front liquid crystal cell and therear liquid crystal cell, wherein a distance between the diffusion layerand the front liquid crystal cell is less than a distance between thediffusion layer and the rear liquid crystal cell, and circuitryconfigured to: convert a color image signal for the display panel into ablack-and-white image signal; generate a rear image signal based on theconverted color image signal for the rear liquid crystal cell; andconvert the color image signal based on a lower limit value of agradation value set for each color included in the color image signal.2. The display apparatus according to claim 1, wherein the diffusionlayer is arranged to be adjacent to the front liquid crystal cell. 3.The display apparatus according to claim 1, further comprising: at leastone of: a first transparent material layer between the diffusion layerand the front liquid crystal cell; and a second transparent materiallayer between the diffusion layer and the rear liquid crystal cell. 4.The display apparatus according to claim 1, further comprising: abacklight arranged on a rear side of the rear liquid crystal cell,wherein the backlight includes a light emitting unit, the light emittingunit including a light emitting diode (LED) configured to emit light. 5.The display apparatus according to claim 1, wherein: the display panelincludes a light source; and an arrangement order of the display panelis the light source, the rear liquid crystal cell, the diffusion layer,and the front liquid crystal cell.
 6. The display apparatus according toclaim 1, wherein the circuitry is configured to perform gradation valueconversion on the black-and-white image signal to generate a convertedsignal for performing gradation expression in a gradation region set asa gradation range.
 7. The display apparatus according to claim 6,wherein the circuitry is configured to perform arithmetic processing onthe color image signal based on the rear image signal to generate afront image signal for the front liquid crystal cell.
 8. The displayapparatus according to claim 6, wherein the circuitry is configured toperform the gradation value conversion according to a lookup table inwhich an output value is stored for an input value.
 9. The displayapparatus according to claim 8, wherein the lower limit value of thegradation value set for each of red, green, and blue is a gradationvalue of the respective one of red, green, and blue at whichchromaticity changes to a predetermined value in a low gradation region.10. The display apparatus according to claim 6, wherein: the color imagesignal includes gradation values of red, green, and blue; and thecircuitry is configured to convert the color image signal into theblack-and-white image signal by performing calculation usingcoefficients of red, green, and blue, each of the coefficients beingcalculated by using a lower limit value of a gradation value set foreach of red, green, and blue.
 11. The display apparatus according toclaim 1, wherein the circuitry is configured to: multiply the rear imagesignal by a correction coefficient corresponding to a light amountcomponent incident on the front liquid crystal cell; and generate afront image signal by dividing the color image signal by the rear imagesignal multiplied by the correction coefficient.
 12. The displayapparatus according to claim 1, wherein a target input gradation is setas a maximum gradation value in a low gradation region in which humaneyes hardly perceive a change in chromaticity.
 13. A display apparatuscomprising: a display panel in which a display image is generated bylight passing through a rear liquid crystal cell and a front liquidcrystal cell, and including a diffusion layer arranged between the frontliquid crystal cell and the rear liquid crystal cell, wherein a distancebetween the diffusion layer and the front liquid crystal cell is lessthan a distance between the diffusion layer and the rear liquid crystalcell, and circuitry configured to: convert a color image signal for thedisplay panel into a black-and-white image signal; generate a rear imagesignal based on the converted color image signal for the rear liquidcrystal cell; multiply the rear image signal by a correction coefficientcorresponding to a light amount component incident on the front liquidcrystal cell; and generate a front image signal by dividing the colorimage signal by the rear image signal multiplied by the correctioncoefficient.